Honeycomb matrixes made from high temperature steel foil are used as support structures for catalytic coatings, for both automotive and industrial (stationary engine) applications. Industrial applications pose different challenges than automotive applications to the service life of the catalyst substrate. This is because of the significantly larger size of industrial type catalytic converters.
The matrix is usually formed by winding previously corrugated foil into a spiral shape to form a multitude of channels or passages. The foil is quite thin, typically on the order of a few thousands of an inch and accordingly relatively easy to bend. In the case of industrial sized units the diameter of the matrix may approach six feet (2.0 m).
The matrix has an axis about which the spiral winds. The passages run generally parallel to the axis. The matrix is mounted within a housing. Although the matrix may be mounted with its axis vertically aligned, in practise the matrix is generally mounted with its axis aligned horizontally with a bottom portion of the outer periphery of the matrix resting on an interior wall of the housing. The balance of the outer periphery is in close proximity to the interior wall to avoid gas leakage about the matrix.
In larger sized converters, failures due to collapse of the channels or passages arise. Contributing factors to the collapse may be the weight of the matrix and thermal stresses. Failure is believed to occur in stages. In a first stage some of the lowermost channels collapse causing the matrix to drop in the housing and enlarge the gap between the uppermost regions of the matrix and the corresponding portion of the interior wall of the housing. The enlarged gap in turn permits gas flow leakage between the housing and the matrix. The gas flow leakage in turn causes the matrix to flutter thereby incurring more damage until it becomes ineffective.
In very large reactors, the matrix is built up of arrays of smaller rectangular elements which are shrouded about the perimeter in order to retain the foil and provide a well-defined cross-section. In view of the relatively modest size, the individual elements are not designed with weight bearing or thermal expansion considerations in mind. The present invention is directed at large round cross-section matrixes (rather than built up matrixes) where weight in the past has been supported over a relatively small contact area by the lowermost foil layers. The expression “round section” is intended to reflect the most likely and common design choice rather than to impose a limitation that the cross-section must be circular rather than having another curved profile not perfectly circular.
Matrix life is also a function of how long the catalytic coating deposited thereon will last. This is generally however a function of the amount of coating applied. As the catalytic materials in the coating are very expensive (such as platinum) currently the amount of the coating applied is related to the expected service life of the support structure. If greater longevity were achievable in the support, longer service of the matrix would be achievable by applying more catalyst. While this would increase the cost of the converter it is believed that any such increase would be outweighed by costs associated with the downtime required to exchange the matrix within the converter or to exchange the entire converter.
It is an object of this invention to provide a catalyst substrate support arrangement which is less prone to collapsing than the prior arrangements. It is also an object of this invention to provide a catalyst substrate mounting arrangement which is more tolerant to radial collapse before the onset of leakage than prior designs.